ncm-xrootd: declare $XROOTD_SYSCONFIG_FILE as an exported variable (for testing)
ncm-xrootd: make Configure() testable
(involves adding a new methode configureNode() that allows to fake the host name)
ncm-xrootd: add a test for #28
ncm-xrootd: fix disk instance not added to the list of managed instances

Fixes #28